An optical lithographic fabrication system is used in prior art for delineating features in a workpiece. Typically the workpiece comprises a semiconductor wafer (substrate), together with one or more layers of material(s) (not shown) located on a top major surface of the wafer.
During operation of the optical lithographical system, typically monochromatic optical radiation of wavelength .lambda. is emitted by an optical source, such as a mercury lamp. This radiation propagates successively through an aperture in an opaque screen, an optical condensor such as an optical condensing lens (or condensing lens system), a lithographic mask or reticle, and an optical imaging lens or imaging lens system. The optical radiation emanating from the reticle is focused by the imaging lens onto a photoresist layer located on the top major surface of the workpiece. Thus, the pattern of the mask--that is, the pattern of its transparent and opaque portions--is imaged on the photoresist layer.
The mask or reticle illustratively includes a uniformly thick, optically transparent substrate, typically made of glass. On the underside (bottom surface) of this transparent substrate is located a patterned opaque layer, typically made of chromium, having two illustrative portions: (1) an illustrative portion which is suitable for forming an image comprising an isolated circular or an isolated square bright spot (localized area) on the photoresist layer, and (2) another illustrative portion which is composed of mutually parallel opaque stripes suitable for forming an image comprising mutually parallel lines and spaces on the photoresist layer. The illustrative portion which is suitable for forming the isolated bright spots typically is composed of an opaque solid layer having an isolated circular or isolated square aperture in it.
Depending upon whether the photoresist layer comprises negative or positive resist material, when it is subjected to a development process, typically a wet developer, the material of the photoresist respectively remains or is removed at, and only at, areas on which the optical radiation was incident. Thus, the pattern of the mask is transferred to the photoresist layer, whereby a patterned photoresist layer is formed.
Subsequent etching processes, such as wet etching or dry plasma etching, can then remove portions of the workpiece in accordance with the pattern of the photoresist layer and hence in accordance with the pattern of the mask. That is to say, portions of the workpiece are removed from the top surface of the workpiece at areas underlying those regions where the material of the photoresist layer was removed by the development process but not at areas underlying those regions where the photoresist remains. Alternatively, instead of etching the workpiece, impurity ions can be implanted into the workpiece at areas underlying those regions where the photoresist was removed by the development process but not at areas underlying those regions where the photoresist remains. In accordance with yet another alternative, such as a "lift-off" process, metallization can be deposited or otherwise formed on the workpiece only at areas underlying those regions where the photoresist was removed. Thus, in any event, the pattern of features of the mask--i.e., each feature of the mask such as the above-mentioned parallel lines and spaces as well as the isolated aperture--ultimately is transferred to the workpiece. Such pattern transfer is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its transverse width--or of an aperture in an insulating layer which is to be filled with metal, for example, in order to form electrical connections between one level of metallization and another or to form metallization lines.
When the size of a feature on the mask is made to be so small that the imaging lens system collects the zero'th order of the diffraction pattern of the feature but collects relatively little of the first order of the diffraction pattern of the feature, then a problem arises in that the edges, including corners, of the image of the feature on the photoresist become indistinct, and therefore very small features undesirably are not printed. Hence the optical contrast of the feature as focused as an optical image on the photoresist layer and transferred to the workpiece deteriorates. As is well known, this lower optical contrast results in a poorer resolution of the feature by the photoresist layer. In practice, this problem typically arises when the size of the feature on the mask is smaller than approximately (0.7).lambda./(NA), where NA is the numerical aperture on the mask side ("object side") of the imaging lens. Typically this NA is approximately equal to 0.1 in case the magnification m is in the approximate range of 0.20 to 0.25. Also, the depth of focus of features of the mask as imaged on the photoresist layer is often limited to values that can be lower than are desirable from the standpoint of desired resolution.